rename battle_bg.s

ld_script.txt

@@ -65,7 +65,7 @@ SECTIONS {
         src/main_menu.o(.text);
         src/battle_controllers.o(.text);
         src/decompress.o(.text);
-        asm/battle_1.o(.text);
+        asm/battle_bg.o(.text);
         src/battle_main.o(.text);
         src/battle_util.o(.text);
         src/battle_script_commands.o(.text);